I want to "green" my PC

My primary PC is in my .sig: Q9450 with 4GB of RAM and a HIS HD4850. I've been thinking about how I use it though. It is not as impressive as some other rigs around here, but it's overkill for what I do. Other than common household stuff, the games I play most are Guild Wars and Sacred 2, at 1680x1050. I don't think either of those really need the horsepower, so I've been thinking of paring it down. I've taken the OC off my CPU, which hasn't mattered in the games, and I can tolerate the visible performance loss elsewhere. I want to be less wasteful.
I have a 4850e on a 790GX that can run Guild Wars at 1440x900, but not quite as well as I'd like, so I need more than that.
Given that I'd keep the HD4850 on a shelf for when I really might need it, a 4670 looks like the way to go for now, and may be the only change I make. If I can use a lot less power elsewhere, I'd consider other changes too.
So, what is the minimum STOCK CPU I would need to play those games maxed at 1680x1050? How much LESS power would it use compared to my Q9450? I say stock, because a high OC may defeat the whole power-saving idea. An e5200 at stock would be too muttly, but what about rebuilding with an AMD 720BE (or even 7750X2) on a 790GX? The PSU would not change; it is 80+ Bronze and AFAIK, I would be unable to find a better one today.

Your thoughts please?
I admire your intentions.

However, if you buy another CPU and another GPU you will in fact cause more pollution than you save, because the new CPU and GPU need to be produced and wrapped up and shipped. Also, it would cost you $100 or more to buy them, while the energy savings will be only a few dollars a year.

How about just donating $100 to some worthy organization (something about the environment, or a nature museum or whatever). That would be good for the environment too, and you might even get part of the money back in your tax refund.
I was thinking mostly in terms of reduced energy usage; it isn't really an economic decision, although thinking only in those terms is one of the things that has led our society to be be so wasteful with energy use in the first place.
I like the idea of downclocking, particularly of the GPU. I will do some testing with that I think. Getting down to 14W in 2D seems unlikely, but still probably worthwhile.